The present invention relates in general to position sensing for electric machines used in electric vehicle drives, and, more specifically, to an excitation signal used to drive a resolver in an electric drive system.
Electric vehicles, such as hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), use inverter-driven electric machines to provide traction torque and regenerative braking torque. A typical electric drive system includes a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a variable voltage converter (VVC) to regulate a main bus voltage across a main linking capacitor. A first inverter is connected between the main bus and a traction motor to propel the vehicle. A second inverter is connected between the main bus and a generator to regenerate energy during braking to recharge the battery through the VVC. As used herein, electric machine refers to either the motor or generator.
The inverters include transistor switches (such as insulated gate bipolar transistors, or IGBTs) connected in a bridge configuration. An electronic controller turns the switches on and off in order to invert a DC voltage from the bus to an AC voltage applied to the motor, or to invert an AC voltage from the generator to a DC voltage on the bus. In each case, the inverters are controlled in response to various sensed conditions including the rotational position of the electric machine.
Position signals are typically sensed using a device known as a resolver. One preferred type of resolver for use in an electric vehicle is the variable reluctance resolver. A resolver produces a signal that provides an absolute indication of the rotational position of the rotor of the electric machine to which it is attached. The resolver has a rotor and stator, wherein the resolver's rotor is connected to the rotor of the electric machine in order to rotate together. The resolver has a primary winding that receives an excitation signal comprised of an AC voltage. A pair of secondary windings of the resolver each generates an induced voltage that depends on the position of the rotor. A ratio of the induced voltages provides a measure of the rotational position of the electric machine.
A specific frequency used for the excitation signal can be chosen within a fairly wide range of frequencies. Integrated circuit products known as a resolver-to-digital (R2D) chip are available that generate an excitation signal and process the induced voltages to produce a digital representation of the rotor position. For example, the AD2S 1200 is a 12-bit R/D converter with reference oscillator available from Analog Devices of Norwood, Mass., which offers a selection of a fixed excitation frequency at either 10 kHz, 12 kHz, 15 kHz, or 20 kHz.
The electric drive system is subjected to significant amounts of electromagnetic noise propagating to and from the motor and/or generator. Since the resolver is mounted very close to the electric machine on the same shaft, it is especially sensitive to electromagnetic noise coupling from the electric machine. This noise can result in incorrect position feedback, potentially causing unstable operation and increased losses. The addition of electromagnetic shielding components is undesirable because of the added weight, cost, and space penalties. A better signal-to-noise ratio can be obtained by generating the resolver excitation signal at a higher voltage amplitude, but this solution also results in increased cost and additional space on the printed circuit board in the control module.